Hydraulic hoses and pipes contain steel fittings to enable hoses to be coupled together, to be attached to another member (such as an outlet) and the like. These fittings can be broadly classed into male fittings and female fittings. The male fittings contain external threads and are designed to connect to female fittings. Conversely, the female fittings typically contain an internally threaded recess.
The recess in the female fittings typically contains an innermost sealing surface that may be flat or tapered. The internal threads are positioned between the sealing surface and the open end of the recess. This type of coupling is well-known. However, there is a large range of this type of coupling where the sealing surfaces are different, and the pitch of the thread is different.
Occasionally, it is necessary to plug an otherwise open fitting. However, it is common for the open fitting to leak hydraulic fluid, which can be quite toxic and corrosive. Even if the line has been drained of hydraulic fluid, there may still be an amount of fluid present in the line which can leak through the open fitting. Thus, a variety of plugs is known in the marketplace. It is known to provide a hard plastic threaded plug that screws into the female fitting. The plug contains a tapered nose that engages against the tapered sealing surface. A disadvantage with this arrangement is that a particular plug can only be used in a particular female fitting having the same thread pitch, and, as there is a large number of different types of fittings, a large number of different types of plugs is required. Another disadvantage is that any damage to the plastic thread of the plug can result in improper sealing and therefore the hydraulic fluid can leak past the plug which is highly undesirable.
It is also known to provide a simple push-in plug. The plug typically contains a small annular ring that can be pressed into one turn of the female thread to hold the plug in place. This type of plug is not very useful as it does not provide any sealing and therefore hydraulic fluid (especially if under some pressure) will easily leak past the plug.
It is also known to provide a snap-fit plug. This plug contains a larger annular rib and is designed to give a tight fit with only a single turn of the plug once the plug is pushed into position. An advantage of this arrangement (and with the push-in plug) is that is not very sensitive to the type of thread and therefore a single plug can be used for a number of threads. The disadvantage of this plug is the same as that of the push-in plug which is that it does not provide any meaningful seal against leakage of hydraulic fluid and the primary use of the plug is to protect the female thread against damage
For male fittings, it is known to provide a threaded protection cap that can be threaded over the end of the male fitting. The disadvantage with this type of cap is similar to that of the hard plastic cap, in that different caps are required for different threads.
It could be argued that one possible solution would be to provide a plug having a soft wall that will cut its own thread when inserted into a female fitting. An advantage of this could be that the plug could be used on different types of threads. However, the disadvantage is that once the thread has been cut into the plug, the plug can then only be used on fittings that have that precise thread and therefore this does not really provide any meaningful solution to the above disadvantages. Also, the plug body would need to have quite a soft wall which might make it difficult to screw the plug in tightly into the recess.
Therefore, there would be an advantage if it were possible to provide some form of plug that could be used on different types of fittings and which is largely independent of the type of thread in the fitting, and which could also provide a useful degree of sealing (as opposed to just protecting the thread) to prevent leakage of hydraulic fluid when the plug is attached, and where the plug could be reused many times on different types of fittings and not just on the same fitting every time.
Another disadvantage with many of the existing plugs, and which has been described generally above, is the requirement to have many different sizes of plugs. Attempts have been made to reduce the number of plugs generally by having a “male” plug that is somewhat adaptable, and a “female” plug that is somewhat adaptable. There would be an advantage if it were possible to provide a plug that could be used both as a male plug and as a female plug.
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